1. Field of the Invention
The present invention is related to power conversion system and, more particularly, to a multiphase switching regulator.
2. Description of Related Art
A buck converter, or step-down switch mode power supply, may also be referred to as a switch mode regulator. Buck converters are often used to step down the voltage in a given circuit. Buck converters receive a high direct current (DC) voltage source and, accordingly, output a lower desired DC voltage.
Popularity of the buck converter is due to its high efficiency and compact size. The buck converter can be used in place of bulky linear voltage regulators at high voltage inputs.
Linear voltage regulators tend to be inefficient. Often, the power devices used in linear voltage regulators must dissipate a large amount of power. Consequently, the linear regulators must be cooled, either by mounting them on heat-sinks or by forced-air cooling (e.g., a fan), resulting in the loss of efficiency. In applications where size and efficiency are critical, linear voltage regulators are outdated and cannot be used.
A buck converter overcomes some of the drawbacks of linear regulators. Buck converters are more efficient, as they tend to have an efficiency rating of 80% or better. Moreover, buck converters can be packaged in a fraction of the size, as compared to linear regulators.
Conventional buck converters, as depicted in FIG. 1, often can include one or more switches, which can be implemented by MOSFETs (Metal-Oxide-Semiconductor-Field-Effect-Transistors). The switches, which are commonly controlled by a multiphase oscillator, can alternate between connecting and disconnecting a voltage source to circuits that drive the desired output. Hence, the duty cycle of the switching determines the output voltage. In addition, a pulse-width modulation (PWM) circuit commonly controls the switching with each switch receiving a different phase of the PWM over the complete period of the oscillator frequency.
Typically, buck converters are useful, and frequently used, in high current applications, for instance, high power microprocessors, Pentium® II applications, Pentium® III applications, notebook computers, desktop computers, network servers, large memory arrays, workstations, DC power distribution systems, and the like. These applications usually require from 10 Amperes (A) to 300 A of current.
Buck converters can include multiple parallel channels to process one or more of the voltage sources to drive a common output. Each channel can be substantially similar and often includes at least one inductor. The input terminal of the inductor is switched between the voltage source and ground.
The DC current through each inductor is proportional to the duty cycle of its PWM signal and the value of the voltage source. Each inductor has a current limit. Typically, more PWM circuits are used when more output current is desired. The output terminals of all the inductors are electrically connected to provide a single output of the power conversion circuit.
The output terminals of all the inductors are tied together and therefore have at least approximately identical voltages. The input terminal of each inductor has a rectangular wave voltage signal, which is derived from the voltage source and ground. The duty cycles of the rectangular wave voltage signals of respective channels are affected by variations in the respective PWM circuits and switches (e.g., design tolerances, offsets, and timing variations). A slight difference in the duty cycle can produce a significant difference in the DC current through the inductor in each channel.
In a multiphase buck converter arrangement, the PWM circuit can provide a variable duty cycle signal to control the switching of each channel. The PWM channels are synchronous with different phases for each channel. Thus, the PWM channels enable each channel to be switched on a different time. The multiple phases increase an output ripple frequency above the fundamental channel switching frequency. Additionally, the input ripple current is reduced, thereby significantly reducing the input and output capacitors, which can tend to be large and expensive.
High efficiency power conversion circuits typically use inductors with low core loss (e.g., ferrite inductors). When the peak design current is exceeded (i.e., saturation), the inductance of ferrite core material collapses abruptly, which results in an abrupt increase in inductor ripple current and output voltage ripple. Thus, it is important to keep the inductor core from saturating. Prior art systems sense the current in each channel and adjust the respective duty cycles to produce the same current for each channel.
An exemplary prior art device is the Texas Instruments® high-frequency, multi-phase controller, i.e., Part No. TPS40090. This controller employs a peak current mode control scheme, which naturally provides a certain degree of current balancing. With current mode control, the level of current feedback should comply with certain guidelines depending on a duty factor, known as slope compensation, to avoid sub-harmonic instability. This requirement can prohibit achieving a higher degree of phase current balance. To avoid the problem, a separate loop that forces phase currents to match is added to the control scheme. This effectively provides a high degree of current sharing independent of properties of a small signal response of the controller. Indeed, high-bandwidth current amplifiers can accept as an input voltage either current-sense resistors, a DCR voltage of an inductor (the inherent resistance in the metal conductor of an inductor) derived by a resistor/capacitor network, or thermally compensated voltage derived from the DCR, DC resistance, of the inductor. This device is limited to four phases and uses a resistive divider to set output voltage.
Another exemplary prior art device is the Intersil® microprocessor CORE voltage regulator precision multi-phase BUCK PWM controller for mobile applications, i.e., Part No. ISL6219A. A benefit of multiphase operation is the thermal advantage gained by distributing the dissipated heat over multiple devices and a greater area. By implementing more than one device, the complexity of driving multiple parallel and the expense of using expensive heat sinks and exotic magnetic materials are avoided. To fully realize the thermal advantage, it is important that each channel in a multiphase converter be controlled to deliver about the same current at many load levels. Intersil® multiphase controllers attempt to guarantee current balance by comparing each current channel to the over-current delivered by all channels and, accordingly, making an appropriate adjustment to each pulse width channel based on this error. The Intersil® device is limited to a three phase arrangement and uses external resistor dividers.
What is needed is a power conversion system, e.g., a buck converter, which can lower an input voltage down to a lower voltage without external resistor dividers. Moreover, a comprehensive and complete buck converter without external resistor dividers and external FETs is needed, which is capable of both a single phase and a multiphase arrangement. It is to such a device and system that the present inventions are directed.